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Settling on setpoints

Determining the best differential pressure between the setpoints presents an interesting problem. The wider the setpoints, the farther the system gets from optimum performance. Many service people set the controls at 10 psid or more to reduce potential cycling. The results are considerable pressure fluctuation and poor performance. By increasing the differential, there is less capacity per unit of pressure — which reduces the rate of response. Effectively using control storage will narrow the operating range of the compressor or compressors. Managing the operating range closer to optimum reduces the energy required without causing excessive cycling.

With centrifugal compressors, the market has been attracted to the concept of holding a single control pressure. For this to occur, the unit must be operated in modulation. Also, a misconception exists that this type of compressor does not need storage. Although there is some truth to this statement, there are a few problems associated with no capacity to store air and the use of single setpoint modulation. One is the need to always have more compressor capacity on line, either throttled or blowing off, so that negative event changes in the system can be supported without a significant drop in pressure. On the other hand, if a large demand event exits the system faster than the response speed of the compressor control valves, the compressor can rise to surge without adequate storage. Too frequently, the setpoint adjustment is set low in modulation to compensate. The results can be poor efficiency. Controls should not be used to compensate for poor system design. When the controls are slow to respond, the compressors are either too large for the event or there is insufficient storage.

The ability to accurately hold setpoints in the controller will influence the differential between the load pressures of each compressor. This differential will determine how far into draw down a compressor must go before the next compressor loads or is influenced by the signal. It will also determine the lowest pressure which results in the system at the highest demand volume.

As an example, assume a system has four compressors (each with a 10 psig deadband) and the system's pressure switch drifts up to 3 psig. There would be 10 psi for the first base load unit and at least 3 psi for each additional compressor — more than 16 psi across the entire volumetric capacity of the compressors. Using less than 3 psid between load pressures means that the inherent drift in the controller could cause the compressors to fight each other, Figure 6.

Another consequence of too high a differential between load setpoints is that as more volume is required and the pressure drops, all of the base-loaded compressors will pump their capacity at lower pressures. Depending on where the deadbands are relative to optimum performance, this can mean considerable loss of mass per kilowatt for all units that are on. There have been systems where the setup of the deadbands was so inappropriate that the base load capacity would drop faster than the trim compressors could add capacity. The result was an inability to hold pressure on larger events. In this case, there was enough capacity, but the setup used gave the appearance of insufficient supply.

The accuracy of the pressure signal will also influence the results. Microprocessor controls receive an electronic signal from a pressure transducer. Most manufacturers use 62% accuracy; if the transducer is rated at 0 to 200 psig, there can be a 64 psig error across the range of use. This could cause erratic loading of an additional compressor — or make it difficult to operate a compressor or the whole system.

With centrifugal compressors, the signal problem can cause hunting of the inlet and blow off control valves as well as difficulty with more than one machine. Consequently, the use of more accurate devices is strongly encouraged. In higher pressure compressors, the accuracy of the control signal and receiver becomes more important. Improved accuracy costs more money, but it is insignificant compared to the demand and usage electric charges of operating with an extra compressor.

R. Scot Foss is president of Plant Air Technology, Charlotte, N.C., which specializes in air system auditing and design. This series of articles is based on his book, Compressed Air System Solution Series. To order a copy, click here.